1. Field of the Invention
This invention relates to an optical fiber transmission-line for transmitting multiple wavelength optical signals in a wavelength division multiplexing transmission system.
2. Related Background Arts
A wavelength division multiplexing (WDM) transmission system can effect high speed, high capacity optical communication by transmitting multiple wavelength optical signals. Because the transmission loss of a silica optical fiber used as an optical fiber transmission-line is small around the wavelength 1.55 xcexcm, and optical amplifiers for amplifying optical signals in the wavelength 1.5 xcexcm band are available, multiple wavelength optical signals in the wavelength 1.55 xcexcm band are used in WDM transmission systems.
When in an optical fiber transmission-line for transmitting multiple wavelength optical signals there is chromatic dispersion in the optical signal wavelength band, the pulse waveform of the optical signal collapses and transmission quality deteriorates. Therefore, from this point of view, it is desirable for the chromatic dispersion in the optical signal wavelength band to be small. On the other hand, when the chromatic dispersion in the optical signal wavelength band is substantially zero, the nonlinear optical phenomenon of four-wave mixing tends to occur, crosstalk and noise arise, and transmission quality deteriorates. The occurrence of four-wave mixing can be suppressed by making repeater spans short and reducing optical signal power, but because this makes it necessary to provide many optical amplifiers, it results in a generally expensive optical transmission system.
To deal with such problems, dispersion-management has been proposed, wherein sections where the chromatic dispersion at the wavelength 1.55 xcexcm is positive and sections where it is negative are provided alternately in the longitudinal direction of the optical fiber transmission-line. If this kind of optical fiber transmission-line is used, by making the average chromatic dispersion in the optical fiber transmission-line as a whole substantially zero, it is possible to suppress transmission quality deterioration caused by chromatic dispersion. And because at most points in the optical fiber transmission-line the absolute value of the chromatic dispersion is not in the vicinity of zero, it is thought to be possible also to suppress transmission quality deterioration caused by four-wave mixing (see for example U.S. Pat. Nos. 5,894,537 or 5,887,105).
However, the present inventors have recognized that in this related art technology, when high-speed signal transmission with a bit rate exceeding 10 Gb/s is carried out using an optical fiber transmission-line wherein the alternating disposition of the sections where the chromatic dispersion is positive and the sections where it is negative is regular, side bands form around the original wavelength of the optical signal. This formation of side bands appears to be caused by the interaction in the same pattern between the optical signal spectrum and the chromatic dispersion as a result of the regularity of the alternating disposition. And because this formation of side bands constitutes a cause of transmission quality deterioration, it is important that it be suppressed.
It is therefore an object of the present invention to provide a dispersion-managed optical fiber transmission-line with which the formation of side bands around the optical signal wavelength can be suppressed even when carrying out high-speed signal transmission.
To achieve this object, a first optical fiber transmission-line according to the invention is an optical fiber transmission-line forming a single repeater span in which sections where the chromatic dispersion at a predetermined wavelength is positive and sections where it is negative are provided alternately, wherein the ratio between the maximum value and the minimum value among the absolute values of the average chromatic dispersions of the sections is not less than 1.3 and not greater than 10.0.
A second optical fiber transmission-line according to the invention is an optical fiber transmission-line forming a single repeater span in which sections where the chromatic dispersion at a predetermined wavelength is positive and sections where it is negative are provided alternately, wherein the number of sections of which the absolute value of the average chromatic dispersion differs by not less than 10% from that of an adjacent section is at least half of the total number of sections.
A third optical fiber transmission-line according to the invention is an optical fiber transmission-line forming a single reperter span in which sections where the chromatic dispersion at a predetermined wavelength is positive and sections where it is negative are provided alternately, wherein the number of sections of which the absolute value of the average chromatic dispersion differs by not less than 0.5 ps/nm/km from that of an adjacent section is at least half of the total number of sections.
A fourth optical fiber transmission-line according to the invention is an optical fiber transmission-line forming a single repeater span in which sections where the chromatic dispersion at a predetermined wavelength is positive and sections where it is negative are provided alternately, wherein for any two sections the absolute value of the average chromatic dispersion of the section nearer the optical signal input end of the repeater span is larger than the absolute value of the average chromatic dispersion of the section nearer the optical signal output end and the absolute value of the average chromatic dispersion of the section at the output end is not less than 1 ps/nm/km. For any two sections in this. fourth optical fiber transmission-line, the length of the section nearer the optical signal input end of any repeater span is preferably shorter than the length of the section nearer the optical signal output end.
With any of the first through fourth optical fiber transmission-lines described above, the formation of side bands around the optical signal wavelength can be suppressed even when carrying out high-speed signal transmission, and even when carrying out high-speed signal transmission with a bit rate exceeding 10 Gb/s it is possible to realize WDM transmission of good transmission quality.
Also, with the fourth optical fiber transmission-line described above, it is not necessary for the sign of the chromatic dispersion to be frequently alternated in regions where the absolute value of the chromatic dispersion is large, and the manufacturing productivity of the optical fiber transmission-line is thereby improved.
And, in each of the first through fourth optical fiber transmission-lines described above, when the length of each section is not less than 0.1 km and not more than 10 km, the cumulative chromatic dispersion does not become large, so the deterioration of transmission quality caused by the interaction of cumulative chromatic dispersion and nonlinear optical phenomena can be suppressed. When the absolute value of the average chromatic dispersion of each section is at least 1 ps/nm/kn, transmission quality deterioration caused by nonlinear optical phenomena can be suppressed. When the absolute value of the average chromatic dispersion of the whole repeater span is 0.5 ps/nm/km or less, transmission quality deterioration caused by cumulative chromatic dispersion can be suppressed. When the polarization mode dispersion of the whole repeater span is not greater than 0.2 ps/km/xc2xd, transmission quality deterioration caused by polarization mode dispersion can be suppressed. When the transmission loss is not more than 0.3 dB/km, the distance of a transmission line without repeaters can be made greater. And when the effective core area is at least 20 xcexcm2 over the whole repeater span, transmission quality deterioration caused by nonlinear optical phenomena can be suppressed.
In this invention, a xe2x80x9cpredetermined wavelengthxe2x80x9d means the center wavelength of an optical signal wavelength band, for example, 1.55 xcexcm. And, when not otherwise specified, values of polarization mode dispersion, transmission loss and effective core area are values at the predetermined wavelength.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.